This invention relates to improved reinforced reaction injection molded (RIM) polymeric articles and to a method of making them. More particularly, the invention relates to the incorporation of flaked glass particles in liquid RIM precursor constituents. The constituents are molded such that the glass flakes are preferentially oriented to improve physical characteristics of the polymerized article.
Reaction injection molding (RIM) is a process by which highly chemically reactive liquids are injected into a mold where they polymerize in a few seconds to form a coherent, molded article. The most common RIM processes today involve a rapid reaction between highly catalyzed polyether or polyester polyol and isocyanate constituents. The constituents are stored in separate tanks prior to molding and are first mixed in the mixhead upstream of a mold. Once mixed, they react rapidly to gel and then harden to form polyurethane polymers. While the invention will be specifically described in terms of urethane RIM systems, it has application to reaction injection molding processes based on other very rapidly reacting chemical systems.
Although reaction injection molded urethanes have many desirable physical characteristics, they also have generally high coefficients of thermal expansion (CTE), poor dimensional stability over wide temperature ranges and considerable flexibility at room temperature. Morover, a large, filler-free RIM panel when attached to a rigid support structure may permanently buckle and wave at elevated temperatures. Thus, as molded, unreinforced RIM urethanes are not generally directly suitable for use as large automotive panels or in other semistructural or structural panel applications. Furthermore, the larger the surface area and thinner the aspect of a panel, the more serious these problems become. As a consequence, the use of reinforcing fillers in RIM urethanes has been extensively examined by this inventor and others.
Currently, some automotive body panels are made from RIM urethanes filled with short (less than 1/8") milled glass fiber, generally in amounts less than about 25% of the polymer weight. Chopped glass is not a good filler for RIM systems because it makes the liquid in which it is contained difficult to impingement mix, even at inclusion levels of only a few weight percent.
Wollastonite, a calcium metasilcate-based mineral, is sometimes used as a low cost alternative to milled fiberglass. However, its morphology is the same as that of glass fiber so it creates the same problems in molded panels. I have found that the moisture content of wood fibers is too high for use in isocyanate-containing systems. Latent water reacts to form urea and carbon dioxide which cause high porosity and poor strength in urethane RIM panels. Other isocyanate compatible fibrous fillers such as carbon fibers, asbestos, nylon, rayon, etc., produce results similar to those of milled glass. Therefore, I do not believe that any single type or combination of fibrous fillers alone will provide the increased isotropic strength, decreased CTE and cosmetic characteristics desirable for automotive body and other thin aspect structural panels.
The incorporation of solid and hollow glass spheres to improve the physical properties and appearance of RIM panels was also examined. However, incorporating glass spheres did not result in any significant improvement in strength or reduced coefficients of thermal expansion.
Attempts were also made to incorporate flakeshaped mica particles in urethane RIM panels. However, the mica flakes adversely effected the chemistry of the constituents so that all test panels had very poor adherence and inferior physical properties.
Another problem encountered in the manufacture of large urethane RIM panels is the rapid impingement mixing of the reactive isocyanate and polyol constituents. Unless gelation takes place within a few seconds after mixing and mold injection, the process cannot be used to economically produce parts weighing several pounds at high volumes. The RIM systems that have been used commercially for the last few years gel within two to six seconds. Once gelation occurs, movement of filler particles is restricted.
To get good and rapid mixing, the constituents must be fluid when they are ejected through the impingement mixing ports at high pressures (.about.2000 psi). Today's RIM systems are so fast that by the time the mixed liquids fill the mold, they have already gelled. Obviously, this property further limits the scope of acceptable fillers. It certainly precludes the use of paste-like constituents containing high filler loadings. Such pastes cannot be dispersed into the liquid constituents rapidly enough to assure complete mixing and uniform distribution of filler particles in a uniform molded panel.
Thus, although many different varieties of reinforcing fillers have been examined for urethane RIM, none seem to be capable of providing the desired results. The complexity of the reaction injection molding process itself and the sensitivity and criticality of the rapidly reacting constituent chemicals clearly do not easily accommodate the incorporation of reinforcing fillers in RIM products to improve CTE, isotropic strength, appearance or other important physical properties.